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2 Protocols for Applying Phytotechnologies in Metal-Contaminated Soils 21

feasibility test can be used subsequently to decide whether it is possible or not to
apply phytotechnology in the real field context, and if so, which approach gives the
lowest risk of failure in attaining remediation goals (Koopmans et al. 2008a, b).

2.1.2 The Concept of Heavy Metal Bioavailability and Its
Importance in Phytotechnologies

It is scientifically accepted that the risky fraction of metals are the mobile/
bioavailable fractions, despite the fact that this terminology (especially regarding
bioavailability) is vague and various definitions are given in the last few decades.
It is well-known that during workshops attended by both soil chemists and soil
biologists normally additional definitions are invented. Despite the lack of widely
accepted definitions, the message is clear: the total heavy metal content in a soil
gives no accurate indication regarding risks which are related to the heavy metal
contamination, including phytotoxicity, leaching risks, and uptake by plants
(i.e. food-chain propagation) (Barbafieri et al. 1996; Barbafieri 2000). It can be
boldly stated that elimination/reduction/stabilization of the risky fractions is the
most necessary and valuable action to solve the problems caused by contaminated
soil. The main problem of this statement is the fact that policy makers have to
convince; this might be difficult as many soil quality standards are still based on
total concentrations in the soil. In this chapter, authors will focus on the descrip-
tion of applicability protocols for phytoremediation in heavy metal-contaminated
soils focusing on the importance of “mobile/bioavailable” fractions of heavy
metals in the soil. Despite the numerous articles appearing in scientific journals,
very few field applications of phytoextraction have been successfully realized
until now. To overcome the imbalance between the technology’s potential and its
drawbacks, there is growing interest in the use of plants to reduce only the
fraction that is the most hazardous to the environment and human health, which
is to target the bioavailable fractions of metals in soil.
At a first glance phytoextraction and phytostabilization seem to have a different
goal and, regarding many practical aspects, they indeed do. But despite this, it can
be stated that both approaches aim at reducing the amount of mobile/bioavailable
heavy metal fractions in the soil. In phytoextraction this is done by removing such
fractions and in phytostabilization this is done by reducing heavy metal mobility
and bioavailability without removing heavy metals. In the case of phytoextraction,
the action of plants only targets the mobile/bioavailable fraction unless other
“stronger actions” are taken, e.g., the use of additives to increase the heavy metal
mobility, making them more available for plant uptake. Plants can, using their
absorbing roots, deal only with the “plant-available” fractions, which can them-
selves be manipulated by chemical additives or biological action. Moreover, such
fractions can strongly vary among different plant species and even varieties. Some
plants used in phytoextraction, so-called hyperaccumulators, apparently have the